1. Field of Invention
The invention relates to a silicon pressure micro-sensing device and the fabrication process thereof, which require only a single substrate without any adhesion, and the fabrication for difficulty-free taper chamber meets.
2. Related Art
The history of pressure sensor development using silicon substrate as the base material is long-standing. Its sensing principle is based on the piezo-sensitivity of silicon. That is, when a resistor made by silicon substrate encounters pressure or deformation, the value of resistance thereof changes accordingly. In order to enhance the sensitivity to increasing deformation caused by pressure, a membrane structure is generally constructed to sustain a pressure difference on both ends. A piezo-resistance unit is preferably placed onto the area with the most membrane deformation, to attain the highest degree of sensitivity.
A piezo-resistance sensing unit made from monocrystalline silicon has the following advantages: (1) the sensing unit can be manufactured by semiconductor fabrication; (2) the membrane can be formed by a conventional anisotropic etching technique using bulk micro-machining technology. In addition, if the popular electro-chemical-etching-stop technique is utilized to implement the bulk micro-machining, a P-type silicon can be removed leaving an N-type silicon, thus forming an N-type stress membrane on a surface of the unit; (3) as a result of the membrane and the substrate being made from the same silicon, residual stress is unlikely to be present between the two, therefore the accuracy and yield of the product are both elevated. Micro pressure-sensing devices have for a long time mostly been produced by backside bulk micro-machining.
FIG. 1 shows a structure of a conventional sensing device. The conventional sensing device includes: a substrate 13, a membrane 14, and piezo-resistance sensing units 15. Anisotropic etching that starts upward from the bottom of the substrate 13 and narrows with a particular angle to the membrane 14 forms the conventional sensing unit. The particular angle is formed because the included angle between the inclined crystal plane (111) for etching stop and a horizontal plane is 54.7xc2x0. In accordance with this geometric structure, in order to obtain a larger area of the stress membrane 14, the bottom area of the substrate 13 is necessarily comparatively larger. This results in unnecessary silicon substrate loss in the substrate 13, causing a comparatively low yield; especially for a large-sized substrate that has a larger thickness as well as larger bottom openings, not only is more silicon substrate wasted, but the anisotropic etching process time is also prolonged, and these factors increase the cost.
In order to minimize substrate waste and increase the yield for each wafer, U.S. Pat. No. 6,038,928 discloses a structure formed by a front-side bulk micro-machining process.
FIG. 2 shows the structure of the sensing unit in the U.S. Pat. No. 6,038,928. Its fabrication process includes the following steps: First, a taper chamber 26 is formed on a silicon substrate 23 by conventional anisotropic etching, and then adhered to a P-type substrate 27 with an N-type epitaxial layer by wafer adhesion technology. Next, making use of the electro-chemical-etching-stop technique to etch away the P-type substrate 27 situated on top of the epitaxial layer and leaving an N-type epitaxial membrane 24, obtains an adhered wafer 28 with an inner chamber. Then, piezo-resistance sensing units 25 and metal lines (not shown) are formed by semiconductor fabrication and finally a structure for a pressure-sensing unit is obtained. In order to allow the taper chamber 26 an open access to the exterior; the back of the silicon substrate 23 is etched to form an opening 29. The taper chamber 26 of a die formed by front-side bulk the micro-machining process contracts downwards, therefore the needed supporting area on the sensing unit periphery is consequently much smaller.
Referring to FIG. 3, if the aforesaid membrane is in the shape of a square with each side length thickness of the wafer thickness at 400 xcexcm, and the reserved width at the supporting periphery being 250 xcexcm, the area of a conventional unit with back-side opening is hence approximately 265% larger than that of a unit with front-side opening. Naturally the number of sensing dies output per wafer decreases with the same percentage.
Although the front-side bulk micro-machining process has the advantage of reducing the area of the device, nevertheless, the fabrication process in the U.S. Pat. No. 6,038,928 essentially puts the costly substrate adhesion process into practice, which doubles the amount of wafer usage and implements two copious bulk micro-machining etching processes, with not only the cost increased, but defects during adhesion, which also cause a drop in production yield. In addition, the wafer after adhesion has a chamber structure, which may meet many unpredicted fabrication difficulties when a high temperature fabrication process required for making a piezo-resistance device is further performed.
In view of the need to improve the aforesaid prior art, one object of the invention is to provide a silicon pressure micro-sensing device and the fabrication process thereof, in which a front-side bulk micro-machining process is utilized to make the size of the required dies smaller than those processed by prior art processes.
Another object of the invention is to provide a silicon pressure micro-sensing device and the fabrication process thereof, in which the membrane taper chamber is made by a single substrate and requires no adhesion of two substrates. Thus, the amount of material used is decreased and the fabrication process is simplified so that the cost is significantly reduced.
A further object of the invention is to provide a silicon pressure micro-sensing device and the fabrication process thereof, in which the taper chamber and the sealed membrane are both processed at low temperature. Therefore, it is unnecessary to adjust the conventional fabrication process of piezo-resistance sensing units, leaving no impediments in the fabrication process.
The fabrication process of the silicon pressure micro-sensing device of the invention includes the following steps. First, a P-type (100) substrate with an N-type epitaxial layer thereon is prepared. Then, a plurality of piezo-resistance sensing units, a passivation, and a plurality of device pads are formed on the N-type epitaxial layer. Next, a deep-etching process is performed to the N-type epitaxial layer to form a plurality of holes, which pass through the N-type epitaxial layer to the P-type (100) substrate thereunder. It is followed by performing an etching-stop technique, in which an etchant goes through the holes to remove silicon of the P-type (100) substrate below the N-type epitaxial layer substrate, thus forming a taper chamber in the P-type (100) substrate. Finally, an insulating material is applied onto the N-type epitaxial layer to seal a plurality of holes and attain the desired silicon pressure micro-sensing device.
The silicon pressure micro-sensing device of the invention includes an N-type epitaxial layer which includes: a plurality of piezo-resistance units in the N-type epitaxial layer, for sensing pressure; a passivation layer formed on the N-type epitaxial layer, for preventing inappropriate etching from an etchant in the fabrication process; a plurality of device pads formed on the passivation layer and connected to the leads of an exterior circuit; a plurality of holes, which pass through the N-type epitaxial layer and the passivation so as to act as passages for the etchant; and an insulating membrane on the passivation to seal the plurality of holes. And the silicon pressure micro-sensing device also includes a P-type (100) substrate on a lower surface of the N-type epitaxial layer. It is a substrate with a taper chamber.
The aforesaid and other objects, characteristics, and advantages of the present invention, are illustrated more precisely by the detailed descriptions of the preferred embodiments below.